In recent years, successful methods have been developed for drilling oil and gas wells at underwater locations. As a result, an oil or gas well may be drilled and completed so that the entire wellhead assembly is positioned at a depth at least sufficient to avoid being a navigation hazard to ocean-going vessels (e.g., at or near the ocean floor). Such offshore or underwater wells may be drilled from a vessel, such as a drilling barge, or from a platform mounted on legs extending downwardly to the ocean floor.
A drilling barge or similar vessel is particularly susceptible to movements in response to wave action, even though the barge is anchored. Drilling that is done from a floating vessel must accommodate both lateral and vertical movements of the vessel. Accordingly, drilling equipment such as drilling strings and riser lines, which extend downwardly from the drilling vessel to the ocean floor, must possess a degree of flexibility sufficient to prevent rupture when the drilling vessel moves slightly from its designated location. Typically, the pipe in a drilling string is of a sufficienctly small diameter and has sufficient strength to be flexible enough to avoid damage. The riser line or marine conductor pipe, on the other hand, has a relatively large diameter and encloses the drilling string so that drilling "mud" may be returned upwardly in the annulus between the inner wall of the riser pipe and the outer wall of the drill string. The increased diameter and rigidity of the riser pipe, as compared to the drill string, requires that the riser pipe include at least one coupling or joint assembly that can be readily flexed, can withstand high internal and external fluid pressures, and can hold up under the abrasive action of fluids, well tools and other objects that pass through the riser pipe.
One type of flexible joint used in riser pipes consists of a ball member having a precisely machined spherical surface and a socket member having a complementary, precisely machined spherical surface. The joint is flexed by sliding one of the spherical surfaces relative to the other. Resilient O-rings help seal the joint at the interface between the sliding surfaces. The flexural movement of such a ball joint is impaired, however, when the joint is subjected to high pressures. The joint is also subject to frictional wear and deterioration of both the sliding surfaces and the O-ring seals, which requires frequent repair or replacement of the joint.
Another type of flexible joint for fluid conduits, such as marine riser pipes, utilizes annular flexible elements disposed between flanges secured to adjacent ends of different sections of conduit. The flexible elements comprise alternating layers of a rigid and a resilient material, which are normally metal and an elastomer. The layers or laminations may be annular with flat surfaces, as in the pipe joint of Johnson U.S. Pat. No. 3,168,334, or annular with spherical surfaces, as in the flexible joint of Herbert et al U.S. Pat. No. 3,680,895. Laminated flexible elements permit the necessary flexural movement of a joint and also function as seals. A joint incorporating a laminated element has no "moving" parts and is not subject to the frictional wear encountered with the ball-and-socket joints discussed above. Other flexible pipe joints utilizing laminated flexible elements are described and illustrated in Herbert et al U.S. Pat. Nos. 3,390,899, 3,734,546 and 3,853,337.
While joints utilizing laminated flexible elements avoid the wear problem of convention ball-and-socket joints, the laminated elements have a tendency to rupture and fail upon exposure to high axial loads and high pressure differentials. In particular, the elastomer layers of laminated elements, while capable of carrying high compressive loads, can only withstand relatively low tension loads. Thus, when two adjacent lengths of pipe are subjected to forces that tend to move the lengths axially away from each other, the laminated flexible elements are likely to fail. Efforts to solve the problem of tension loads have included the use of tension bars to carry the tension loads in preference to the laminated flexible elements. Pairs of laminated flexible elements may also be utilized in a joint such that at least one of the flexible elements is always loaded in compression, regardless of the relative axial movement between adjacent lengths of pipe. Reducing or eliminating tensile loads on a laminated flexible element also reduces the likelihood of rupture due to high pressure differentials on the element. Similarly, bonding adjacent laminations into an integral member increases the pressure-resistance of the laminated element.